This invention relates to a high frequency high voltage power supply and, more particularly, to such a power supply that is especially adapted to supply Zener-type loads such as a magnetron for use in a microwave oven.
In order to develop a high frequency high voltage from a low frequency AC supply source, e.g. 115 volts, 60 Hz, it is known to use a current-fed parallel resonant oscillator using a pair of switching transistors operated in a push-pull mode. In this known circuit, a source of DC voltage (e.g. a positive terminal thereof) is coupled to a center tap on the primary winding of a transformer via a series choke coil. A capacitor is connected in parallel with the transformer primary winding to form therewith a parallel resonant LC tuned circuit. The load is coupled across the parallel resonant circuit. A pair of first and second switching transistors (e.g. of the NPN type) have their emitters coupled together to the negative terminal of the DC voltage source and their collectors connected to respective opposite ends (A and B) of the parallel resonant circuit. A secondary winding of the transformer is coupled to the base electrodes of the NPN switching transistors via a drive circuit thereby to alternately switch the transistors on and off to provide a push-pull mode of operation. The DC-AC converter or high frequency oscillator will self-oscillate at a frequency determined by the resonant frequency of the parallel LC tuned circuit.
This prior art circuit provides a simple method of converting DC energy to AC energy. Since the elements of the oscillator comprise transistor switching elements as well as reactive components and a simple drive circuit, conversion efficiencies of greater than 90% can be achieved. Resonant oscillators also reduce potential circuit losses since they minimize switching losses as well. Assuming the inductance of the choke coil is high enough so that the oscillator "sees" a good current source at its supply terminals, Q&gt;.pi. to provide a good quality factor, and ideal switches and reactive components, a sinusoidal voltage waveform of high quality will be generated across terminals A, B of the tuned LC resonant circuit. If the transistors switch alternately at a 50% duty cycle, then the relationship between the RMS output voltage V.sub.AB and the DC input voltage V.sub.cc is given by the expression: ##EQU1## The DC inductor current i.sub.F in the choke coil L.sub.F will be ##EQU2## where I.sub.R is the RMS load current. The load impedance then is reflected to the DC input load impedance R.sub.L as: ##EQU3##
As can be seen from the first expression, the output voltage (V.sub.AB) is independent of the load and is proportional to the input voltage (V.sub.cc), which means the gain is fixed. The switching elements must be designed so as to be capable of blocking at least the peak of the resonant circuit voltage V.sub.AB and must be able to carry the choke coil current, i.sub.F, for one half of the cycle.
In the case of a conduction overlap of the pair of transistor switches during the switching interval, i.e. one transistor has not yet turned off when the other one is turned on, then the admittance is very large and, since it is in parallel with the resonant circuit capacitor (C), it essentially discharges this capacitor. Depending on the degree of conduction overlap, the parallel resonant tank circuit may be heavily loaded or even completely discharged, which in turn may destroy the transistor switching elements.
A second possible case is that of an off-time overlap of the transistor switches during the switching interval, i.e. both switches are off simultaneously. In this case, the admittance reaches a very small value and the current source will force a high voltage across the same to maintain a constant current. Once again, depending on the degree of the switching action, the off-voltage may exceed the voltage rating of the transistor switches and could even lead to a failure thereof.
It is therefore an object of this invention to provide a current-fed parallel resonant oscillator circuit which eliminates the critical overlap conditions of the prior art oscillator circuits and at the same time provides higher voltage gain.
Another object of the invention is to provide a current-fed parallel resonant oscillator circuit in which the voltage gain is dependent on the load.
Consumer microwave ovens conventionally employ a magnetron powered by a ferroresonant power supply operating at the power line frequency to supply microwave heating energy to the cooking cavity of the microwave oven. This power supply is relatively heavy and bulky. The power output of this power supply is discontinuously controlled by means of a control circuit which disconnects the 60 Hz AC supply voltage in order to vary the average microwave heating power applied to a load in the oven cavity. Typically, in a "defrost" or a "keep warm" cycle, the magnetron will be pulsed on for approximately 1 second and will be pulsed off for approximately 10 seconds. As a result of this type of operation, the magnetron heater filament cools down when the magnetron is off. This operation produces stresses on the magnetron which reduce its useful life. It would therefore be preferable to provide a means for varying the average microwave power applied to a cooking load in a smooth and continuous manner so as to keep the magnetron heater filament energized at all times during operation of the oven. It would be advantageous to keep the magnetron heater filament energized without the addition of a separate heater transformer, as in certain conventional magnetron power supplies.